1. Field of the Invention
The present invention relates to a block copolymer. More particularly, the present invention is concerned with a block copolymer comprising at least two S polymer blocks, each comprising vinyl aromatic hydrocarbon monomer units, and one B polymer block or two or more B polymer blocks which contains or collectively contain isoprene monomer units and 1,3-butadiene monomer units, wherein the amount of the vinyl aromatic hydrocarbon monomer units, the total amount of the isoprene monomer units and the 1,3-butadiene monomer units, and the isoprene monomer unit/1,3-butadiene monomer unit weight ratio are within respective specific ranges, and wherein the vinyl aromatic hydrocarbon monomer unit moiety of the block copolymer has a short segment ratio within a specific range, the short segment ratio being defined as the weight percentage, based on the total weight of vinyl aromatic hydrocarbon monomer units contained in the block copolymer, of the vinyl aromatic hydrocarbon monomer units contained in at least one short segment consisting of 1 to 3 vinyl aromatic hydrocarbon monomer units. The block copolymer of the present invention has excellent thermal stability in that, even when the blocks copolymer is heated at high temperatures, the change in the molecular weight distribution of the block copolymer due to crosslinking or breakage of polymer chains of the block copolymer can be suppressed. Therefore, even when the block copolymer is subjected to shaping at high temperatures, occurrence of gelation can be suppressed. By virtue of the above-mentioned excellent thermal stability of the block copolymer, a shaped article (such as a sheet) produced from the block copolymer is free from marked occurrence of fish eyes (hereinafter, a “fish eye” is frequently referred to simply as a “FE”). Further, the shaped article has excellent stiffness, elongation, impact resistance, transparency and heat shrinkability. Accordingly, the block copolymer of the present invention can be advantageously used for producing various shaped articles. The present invention is also concerned with a block copolymer/styrene resin composition comprising the block copolymer and a styrene resin in a specific ratio.
2. Prior Art
A block copolymer comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units (such as a 1,3-butadiene monomer unit) wherein the vinyl aromatic hydrocarbon monomer unit content is relatively high, has various excellent characteristics, such as transparency and impact resistance, so that the block copolymer has been used for producing injection-molded products, extrusion-molded products (such as a sheet and a film) and the like. Further, a composition comprising the block copolymer and a styrene polymer has excellent transparency and mechanical properties, so that the composition has been used for producing a sheet, a film and the like.
With respect to the block copolymer and the composition, a number of proposals have been made in patent documents. For example, in an attempt to improve the transparency and impact resistance of a block copolymer, Unexamined Japanese Patent Application Laid-Open Specification No. 52-58788 (corresponding to U.S. Pat. No. 4,167,545) discloses a branched block copolymer comprising a vinyl aromatic hydrocarbon and 1,3-butadiene, which is obtained by division-wise adding a catalyst to a block copolymerization system. Unexamined Japanese Patent Application Laid-Open Specification No. 53-8688 (corresponding to U.S. Pat. No. 4,120,915) attempts to obtain a block copolymer having an improved thermal stability by a method comprising polymerizing a vinyl aromatic hydrocarbon to obtain a vinyl aromatic hydrocarbon polymer, followed by addition of 1,3-butadiene and a small amount of isoprene to perform a coupling reaction. In an attempt to improve the impact resistance of a block copolymer, Unexamined Japanese Patent Application Laid-Open Specification No. 2-113007 discloses a block copolymer comprising a vinyl aromatic hydrocarbon polymer block and a polymer block composed mainly of isoprene, which has a specific block configuration. For the purpose of obtaining a thermoplastic polymer composition having excellent appearance (transparency, surface luster and the like) and excellent impact resistance, Unexamined Japanese Patent Application Laid-Open Specification No. 58-141233 discloses a composition comprising a vinyl aromatic hydrocarbon/1,3-butadiene block copolymer and a thermoplastic resin, wherein the block copolymer is a mixture of polymer chains which have largely different molecular weights and different compositions. For improving the environmental stress cracking resistance of a block copolymer, Unexamined Japanese Patent Application Laid-Open Specification No. 4-277509 (corresponding to U.S. Pat. No. 5,227,419 and European Patent No. 492490) discloses a method for producing a block copolymer which comprises a vinyl aromatic hydrocarbon and 1,3-butadiene and has gradually varied properties, wherein the method comprises division-wise adding a catalyst to a block copolymerization system. For obtaining a multi-layer sheet made of a thermoplastic resin (a vinyl aromatic hydrocarbon/isoprene block copolymer) which has high stiffness and is improved with respect to rapid shaping property at low temperatures and deep draw shaping property, Unexamined Japanese Patent Application Laid-Open Specification No. 5-177777 discloses a multi-layer sheet comprising a surface layer made of a first resin having a specific elastic modulus and a layer made of a second resin, in which the Vicat softening point ratio of the first resin to the second resin is within a specific range. In an attempt to obtain a block copolymer having excellent transparency and mechanical properties, Unexamined Japanese Patent Application Laid-Open Specification No. 63-145314 (corresponding to U.S. Pat. No. 4,939,208 and European Patent No. 270515) discloses a method for producing a block copolymer having an S-B-B/S-S block configuration, wherein each S independently represents a styrene polymer block, B represents a butadiene polymer block, and B/S represents a butadiene/styrene copolymer block. Further, with a view to improving the transparency and impact resistance of a block copolymer and a composition containing the block copolymer, Unexamined Japanese Patent Application Laid-Open Specification No. 7-97418 discloses a vinyl aromatic hydrocarbon/butadiene block copolymer and a composition comprising the block copolymer, in which the block copolymer has characteristic features with respect to the vinyl aromatic hydrocarbon block ratio, the arrangement of the polymer blocks, the ratio of butadiene in a segment in which a vinyl aromatic hydrocarbon and butadiene are randomly copolymerized, and the like.
However, the conventional block copolymers comprising vinyl aromatic hydrocarbon monomer units and conjugated diene monomer units disclosed in the above-mentioned patent documents have a problem in that the block copolymers have an unsatisfactory thermal stability and, hence, are susceptible to gelation during the heating thereof at high temperatures, so that shaped articles (such as a sheet) obtained from the block copolymers suffer marked occurrence of FE's which are gelled masses of a resin. The occurrence of FE's in a shaped article is disadvantageous, for example, in that, when a print is effected on the shaped article, the print inevitably becomes defective.